The present invention relates to a process for the fractionation and purification of mixtures of aromatic polyamines and to the use thereof.
The preparation of aromatic polyamines and mixtures of aromatic polyamines, especially of the diphenylmethane series, is described in numerous patent applications and patents, as is the use of these products. Of outstanding importance here is the use of these products as raw materials for the preparation of isocyanates, normally by reaction of the polyamine mixtures with phosgene using the generally conventionally and known methods.
In many cases, however, the resulting isocyanates or isocyanate mixtures are not obtained in the form and with the composition that are preferable for further use at the isocyanate stage, but must first be converted to the usable form by working-up and separation processes which are sometimes expensive. Suitable polyamine precursors which can be converted more cheaply to the forms usable for isocyanates are in many cases difficult or impossible to obtain in terms of process engineering or the economics of their preparation are unattractive.
An example is the preparation of the 4,4'-diisocyanatodiphenylmethane important for the manufacture of high-quality polyurethane materials, whose amine precursor can normally be obtained from aniline and formaldehyde only together with isomers, especially the 2,4'-isomer, and higher-functional polyamines. Although these constituents are the basis for isocyanates which are also desirable, separation of the crude isocyanates into the isocyanates or isocyanate mixtures suitable for further use is not easy.
The normal procedure is first to separate some of the dinuclear compounds from the rest. The 4,4'-diisocyanatodiphenylmethane is then separated from the other isomers in the dinuclear fraction in a second distillation step requiring many separation stages.
In more recent times the 2,4'-isomer in enriched form has itself become increasingly important as a polyurethane raw material, but considerable distillation costs are involved in enriching it relative to the 4,4'-isomer and separating it from the 2,2'-isomer which may be present.
Isomer separation processes or enrichment processes within the fraction of the higher-nuclear homologues or higher-functional constituents of the amines, or of the isocyanates of the diphenylmethane series, are practically unknown.
There is also increasing interest in 4,4'-diaminodiphenylmethane as a raw material for di(4-isocyanatocyclohexyl)methane, the ring-hydrogenated form of 4,4'-diisocyanatodiphenylmethane, it being very expensive to prepare suitable mixtures of aromatic polyamines for the hydrogenation stage with the highest possible content of 4,4'-diaminodiphenylmethane and at the same time the lowest possible proportion of 2,4'-diaminodiphenylmethane.
It is known that amines can be separated in certain cases by partial conversion to their salts, utilizing inter alia the different basic strengths. This normally applies to monoamines with very different basic strengths.
Again for mixtures of aromatic polyamines, especially of the diphenylmethane series, such disproportionation effects have already been described in two-phase systems (German Auslegescriften 2,238,319 and 2,528,694). Due to the numerous components present in such a mixture whose amino groups hardly differ in type--practically all are arylamino groups--the effects are not sufficiently great and pronounced to be of interest for direct use with simple agents.
The object of the present invention was to provide a process which made it possible to easily fractionate and purify mixtures of aromatic polyamines to give isomers in pure form or in enriched form.